The superheat is defined as the difference between a temperature of a vapor and the dewpoint temperature of the vapor. In refrigeration systems, it is necessary to provide a superheated fluid to the inlet of a compressor to avoid damage to the compressor. Refrigeration systems are typically designed to provide superheated fluid at the inlet of a compressor which has a superheat of at least several degrees above the dewpoint temperature during operation of the refrigeration system to avoid liquid refrigerant from being provided to the inlet of the compressor during variation of the operating conditions of the system such as changing the opening of an expansion valve providing expanded refrigerant to the evaporator.
Refrigeration systems are typically operated to provide superheat control by either controlling the opening of the expansion valve or the velocity of the compressor to regulate the temperature of the refrigerant being provided to the inlet of the compressor. U.S. patent application Ser. No. 401,866, entitled "Vapor Cycle System With Multiple Evaporator Load Control and Superheat Control", filed on Sep. 1, 1989, which is assigned to the assignee of the present invention discloses a refrigeration system which controls multiple evaporators at different set temperatures with superheat control being provided by regulation of compressor velocity.
Operation of refrigeration control systems to provide cooling of a fluid by an evaporator at a set temperature is known. These systems control the opening of an expansion valve providing expanded refrigerant to the evaporator by computing the difference between the sensed temperature of the cooled fluid and a set point temperature and generating an expansion valve control signal proportional to the difference.
Operation of refrigeration systems to provide a controlled superheat by controlling the opening of an expansion valve is also known. These systems compare the temperature of the refrigerant which is discharged from the evaporator to a set temperature and regulate the opening of the expansion valve to maintain the temperature of the refrigerant entering the compressor within a temperature range typically of several degrees above the dewpoint temperature to prevent liquid refrigerant from being inputted to the compressor which can cause severe damage to the compressor. Inputting of substantial liquid refrigerant into a compressor, especially rotary compressors which run at high rotational velocities, can cause severe damage or destroy the rotor of the compressor.
U.S. Pat. No. 4,112,703 discloses a refrigeration control system in which the temperature of a fluid cooled by the evaporator is maintained at a set point by adjusting the refrigerant temperature at the evaporator outlet in the refrigeration system. The opening of a solenoid controlled expansion valve is controlled by the difference in temperature of the cooled fluid at the outlet of the evaporator and a set temperature which is the desired temperature to control the opening of the expansion valve. The refrigeration system of the '703 patent also controls the temperature of the refrigerant inputted into the compressor to maintain a controlled superheat to avoid liquid refrigerant from being inputted to the compressor. Superheat control is maintained in the '703 patent by comparing the outlet temperature of the evaporator to a set temperature and closing the valve as long as the evaporator temperature is below the set temperature. When the outlet temperature falls below the set temperature, the output of a comparator changes level to cause the solenoid expansion valve to be closed to raise the superheat.
The control of the '703 patent is not controlled by digital logic which limits the parameters of control which are used in controlling the operation of the refrigeration system to provide a controlled set point temperature at the evaporator and a control of superheat. Furthermore, the '703 patent does not provide independent control of superheat and a set point temperature at the evaporator. Operation of the control of the evaporator temperature at a set point is disabled when superheat drops below the minimum level. Superheat control in the '703 patent is controlled only as a function of the temperature at the outlet of the evaporator which limits the degree of control which the system may exercise.
Sophisticated refrigeration systems such as those used in airframes require a number of control parameters to provide the desired system operation. These control parameters are used to limit energy consumption by the system and to provide flexible control for regulating the heat load at a set point temperature. Moreover, it is desirable to operate multiple evaporators at different temperatures to provide control of different heat loads at different set point temperatures. On the other hand, control systems such as disclosed in the '703 patent, which are intended for operation in environments which do not require the sophistication of control found in airframes, do not utilize multiple control parameters including providing control of superheat by varying compressor velocity and control of the heat load at a set point by varying the opening of one or more expansion valves.